Pdf easy method for testing transformer differential relays. This automatic increase in the trip point allows for errors in the relay system that is proportional to the current level. Mitsubishi electric fa site introduces information in latest information, product information, technological material, and the manual, etc. Biaseddifferential protection theory and application eep. Differential relay current differential relay biased. The operation of the protection is based on the biased differential current principle, which is shown in fig. The threephase differential protection incorporated in ref542plus is primarily designed for the protection of twowinding power transformers and highvoltage motors. Protection relays differential relays 2 using biased relay. Use of 87t relay principles for overall differential. The differential relays normally response to those faults which occur in side the differential protection zone of. Siprotec protection relays figure 1 shows a simple differential protection scheme, also known as a merzprice scheme. This is designed to response to the differential current in the term of its fractional relation to the current flowing through the protected section. The presentation explains about the simple differential protection, biased differential protection and also the advanced numerical relay working in generator.
The biased beam relay also called percentage differential relay is designed to respond to the differential current in terms of its fractional relation to the current flowing through the protected section. Strictly speaking the rc tells us how much current a relay will use to restrain based on the currents. The principles of differential protection you must understand on photo. In this simple scheme, we can assume that under normal operating conditions, the current entering into the piece of equipment under protection is equal.
Moreover buchholz relay is provided in transformer for detecting any internal fault in the transformer but differential protection scheme detects the same in more faster way. Ca generator differential relay is a single phase, induction disc type with inverse timing characteristics. The biased differential relay is a lowimpedance relay and compares with the highimpedance relay as follows. Easy method for testing transformer differential relays. Thus, if a numerical 87t differential relay is directly applied for the overall differential protection of a par, and set to compensate for vector group yy0, the false differential current will appear as soon as par phase angle shift is different from zero degrees. In most differential relays, the bias trip point is not a fixed value but calculated from a starting point with a curve that increases as current increases. The operating characteristics of the relay is shown in fig. Differential relays can detect such type of faults. Protection relays manual download mitsubishi electric fa. The bias feature compensates for any ct inaccuracy or saturated cts. In this type of relay, there are restraining coils in addition to the operating coil of the relay. The above disadvantages are overcome to a great extent in biased beam relay. A currentbased solution for transformer differential protection. Differential protection of transformer differential relays.
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